1. Technical Field
The disclosed embodiments relate to waking up a WLAN radio using a BT receiver RF front end.
2. Background Information
Bluetooth (BT) and WLAN (Wireless LAN) are two competing radio technologies that operate using the same 2.4 GHz unlicensed band. WLAN is standardized in IEEE 802.11 and has several variants including IEEE 802.11(n). Bluetooth is a commercial implementation of a Wireless Personal Area Network (WPAN) technology standardized in IEEE 802.15.1.
WLAN and BT, however, differ in that BT is a relatively simple system intended for short ranges in applications that do not require high reliability. WLAN, on the other hand, is more complex and generally more expensive to implement and is intended for larger distances. WLAN involves relatively complex modulation and the energy of the WLAN signal is spread over a wider frequency range of about 22 MHz. The more complex modulation and higher code rate used by WLAN help ensure that WLAN transmissions can be received. A wideband receiver RF front end is therefore required to receive the 22 MHz wide WLAN signals. More complex, and therefore power consuming, demodulation circuitry is also required for WLAN radios. BT, on the other hand, involves relatively simple modulation. The energy of the BT signal is generally confined to a narrower 1 MHz frequency range so a relatively narrow band receiver RF front end can be used. For numerous reasons, WLAN radios generally consume more power than BT radios.
Two types of devices may engage in WLAN communication, an Access Point (AP) and a station (STA). In one common operational scenario, the STA is not attempting to transmit to a WLAN AP, but the STA is out of radio range of the WLAN AP and is in a low power mode referred to as Out-of-Service (OoS). A WLAN AP periodically transmits beacon packets. If an OoS STA were to come into radio range of a WLAN AP, then beacon packets from the WLAN AP would periodically be receivable at the OoS STA. In such a situation, the STA should detect the beacons and begin communicating with the WLAN AP to join the WLAN network. To accomplish this, an OoS STA periodically scans to detect beacons by waking its receiver scan the WLAN channels. There are thirteen such channels. Although the WLAN receiver is inactive for about ninety percent of the time, the WLAN receiver of the STA is active for about ten percent of the time even though the OoS STA is only scanning and is not actually in use. This amounts to a large amount of power consumption. In mobile devices it may be desirable to reduce this power consumption of an OoS STA.
Some types of mobile devices include a BT transceiver in addition to the WLAN transceiver. If a BT transceiver and a WLAN transceiver exist in the same device, the BT and WLAN transceivers are said to coexist. To avoid wasting power, proposals have been made to use the BT receiver to search for WLAN signals. Rather than using the more power hungry WLAN receiver for this purpose, the BT receiver is used. If energy in the 2.4 GHz unlicensed band is detected using the BT receiver, then the WLAN radio is activated to perform subsequent normal WLAN communications. Published U.S. Patent Application US200801081155, for example, describes using a BT receiver to detect WLAN energy. The BT receiver is tuned to several frequencies that are spaced over the WLAN channel frequency band. An energy sample is taken at each frequency and the samples are analyzed to detect WLAN energy. In another example, the BT receiver is tuned to the center frequency of the WLAN channel frequency band. After accumulating the received energy for a length of time, the energy of the sample is compared to a threshold and if the threshold is exceeded then the WLAN radio is activated. Alternative and better ways of using BT receivers to detect WLAN energy are sought.